Systems and methods are known in the art to detect the presence of a person or object at the entrance of an automatic door such as an elevator door or a pedestrian sliding door.
Example of such systems include infrared (IR) and/or microwave detector above the door which registers the increase of frequency of the emitted beam returning to the detector as a person moves towards the entrance, and more generally sense/calculate the motion coming from the objects to be detected.
Safety acoustic sensors are used for powered pedestrian doors, bus and train (transit) door safety, but lack a backup system and are prone to false alarm or lack of sensitivity due to unaccounted reflections and change in the speed of sound.
Furthermore, while an acoustic system can determine the distance to a target, it is ill suited to have sharp angle boundaries, lateral resolution and differentiate the nature of a detected target, whether it is a passenger or a suitcase or to determine the lateral position of the target. Also, due to pour spatial resolution, it does not differentiate well a target size. For example, a small object just under a sensor can give the same signal as a bigger object located at an angle, even though the distance is the same.
More recently, with the increasing power and speed of computer chips, 3D (three dimensional) cameras and other TOF (Time of Flight) based systems have begun to be used in detecting systems for automatic doors.
Examples of such systems are described in the following patent documents:                International Patent Application Publication No. WO 2016/042168 A1, filed on Sep. 21, 2015, naming Harding et al. as the inventors and being titled “Door System with Sensor Unit and Communication Element”;        European Patent Application Publication No. 1 619 469 A1, filed on Jul. 22, 2004, naming Zambon as the inventor and being titled “Laser Scanning Device for Detection Around Automatic Doors; and        U.S. Pat. No. 8,819,992, issued on Sep. 12, 2014 to De Coi et al. and being titled “Monitoring Apparatus and Pivoting Door”.        
Drawbacks of current TOF based detecting systems include the large quantity of data to process, even in single function systems, resulting in either a relatively slow response of the system or the limited precision thereof. The use of a TOF based detection system for safety and/or at least two of safety, activation and/or people counting purposes increases the above-identified problem. Other known drawbacks of such an application include miscounting due to shade, other visual artifacts, and blindness.